PROJECT SUMMARY Influenza remains an important cause of morbidity and mortality and current therapies have limited efficacy. Respiratory failure in influenza results from either prolonged viral replication or lung injury induced by an over exuberant immune response. It follows that a better understanding of the mechanisms that regulate the host immune response to IAV could lead to new therapies. Effector CD8+ T cells are responsible for clearance of influenza A virus (IAV) during primary infection but have also been implicated in immune- mediated lung injury. Effector CD8+ T cells also give rise to influenza-specific CD8+ tissue resident memory T cells (Trm), which mediate heterosubtypic immunity, decreasing the severity of subsequent IAV infection. Thus, the magnitude and quality of the CD8+ T cell response to IAV must be tightly controlled to achieve viral clearance, limit immune-mediated lung injury and promote the formation of tissue-resident memory. Dendritic cells (DC) activate nave T cells in the lymph node to generate an effector CD8+ T cell response, interact with effector CD8+ T cells within infected lung to promote viral clearance and attenuate immunopathology and are required for the generation of optimal CD8+ Trm. Thus, DC play a critical role in regulating both effector CD8+ T cells and in the formation of protective memory. Effector CD8+ T cells are also regulated by inhibitory (i.e. checkpoint) receptors. T cell immunoglobulin and mucin domain 3 (Tim3) is an important checkpoint receptor that is induced on activated T cells and constitutively expressed on DC. Tim3 modulates the immune response to IAV, but it is not known whether it constrains viral clearance or protects from lung injury. We previously demonstrated Tim3 attenuates effector CD8+ T cell responses and protects against mortality in a mouse model of IAV. We now have data demonstrating that Tim3 promotes antigen cross presentation by DC ? a process critical for generating both effector and tissue resident memory CD8+ T cells in IAV infection. Our central hypothesis is that Tim3 determines the balance between viral clearance and lung injury and promotes the formation of CD8+ Trm. In Aim #1, we will test the hypothesis that Tim3 promotes antigen cross presentation by DC and DC-mediated activation of naive CD8+ T cells during IAV infection. Specifically, we will (A) identify the intracellular mechanisms utilized by Tim3 to promote cross presentation, (B) characterize the expression and function of Tim3 on human lung DC, and (C) determine the role of Tim3 on lung DC in activating nave CD8+ T cells during IAV infection. In Aim #2, we will test the hypothesis that Tim3 promotes viral clearance and attenuates immune- mediated lung injury during IAV infection and promotes IAV-induced CD8+ Trm formation. Experiments in this aim will A) define the effect of Tim3 in regulating effector CD8+ T cell responses in the lung during IAV infection and (B) determine the role of Tim3 in IAV-induced CD8+ Trm formation. These experiments are a critical first step toward developing Tim3 as a therapeutic target for influenza.